Heating, ventilation and air conditioning systems for large commercial and institutional buildings such as schools and large offices require elaborate temperature control systems. For many years, pneumatic control systems using pneumatic actuators were popular because they offered the advantages of low cost, ease of installation, maintainability and reliability. A typical pneumatic control system has a centrally-located air compressor. Compressed air is processed through a refrigerated air dryer, a series of filters, and finally a pressure reducing station to provide a system pressure of 15-25 PSI. This low pressure air, sometimes called main air, is distributed throughout the building via transmission lines to various low-pressure pneumatic controllers. The controllers will control the main air pressure through a branch line going to a controlled device or actuator for valves, damper motors and other regulating devices in a heating/cooling system. The controller responds proportionally to variations in a local environment temperature being controlled. Proportional control, as contrasted to intermittent, fully on or fully off, is preferred for commercial applications.
In general, the pneumatic actuator converts the branch line air into mechanical motion by pressurizing and depressurizing a rubber diaphragm or other type of piston assembly which in turn pushes against a piston shaft operatively connected to valves that control heating or cooling liquids or to dampers or damper motors for controlling air movement. As the diaphragm inflates, its size increases and it repositions the piston shaft. Conversely, as the diaphragm deflates, the piston shaft is returned to its original starting position by means of a return spring. Proportional pneumatic actuators provide accurate temperature control. Because they are reliable, low cost and relatively easy to install and maintain, they are still in wide use.
In recent years, computerized temperature controls have increased in popularity because, as compared to pneumatic systems, they offer greater flexibility and programmability for total environmental control of the building. The computer can network essentially every piece of information about the building at a central computer workstation where an operator can quickly diagnose, evaluate and set the temperature control throughout the building. Older pneumatic systems cannot effectively provide this capability. It would be cost-prohibitive and too mechanically complex to run large numbers of air transmission lines from every point of control in the building back to one centralized location. Early computerized systems used proportional electrical-hydraulic actuators at the valves and dampers where the actuator included an electric motor driving a hydraulic pump. Motor speed and, hence, pump output pressure are controlled from the computer.
Electrical-hydraulic actuators are much higher in cost than pneumatic actuators, making installation of a computerized system considerably higher than a pneumatically controlled system. Electrical-hydraulic actuators are relatively expensive not only due to higher initial cost, but also maintenance and replacement cost. The large number of moving parts, such as gears, springs, motors, clutches and limit switch assemblies, usually require hand assembly and are less durable. The motor, gears and clutches run in a housing filled with oil which can leak, often ruining ceiling tile and carpets. Electrical-hydraulic actuators are typically designed to operate in only one orientation, e.g., top side up. Because of their size, shape and limited orientation, electrical-hydraulic actuators may be difficult and expensive to install for some applications and special orientations. For example, two or three linkages and associated brackets might be required to obtain sufficient angular rotation and/or linear displacement at a valve in a manner such that the electrical-hydraulic actuator does not bind or stall. If the actuator is proportional, an elaborate balancing circuit is required along with a feedback circuit, requiring as many as eight to twelve wires per actuator.
Because of the cost disadvantages associated with electric-hydraulic actuators, hybrid systems have also been used where the system is centrally computerized and pneumatic field actuators are used to open and close valves and dampers. This is accomplished by interfacing the computer output to electrical-pneumatic pressure transducers for controlling the position of a pneumatic actuator. The hybrid system uses the same centrally located air compressor as a full pneumatic system and a pneumatic actuator similar to that in the full pneumatic system. The difference is that sensing and controlling is done through a computerized system using electronic signals transduced to corresponding pneumatic signals which in turn are used to actuate valves, damper actuators, and the like. However, electrical-pneumatic transducers are expensive as compared to prior art full pneumatic actuators, but not quite as expensive as electrical-hydraulic actuators. Hence this system is, at best, a compromise in cost and durability between the full pneumatic system and the computerized, electrical-hydraulic actuator systems.
By way of further background to the present invention, semiconductor thermoelectric devices utilizing the Peltier effect are also well-known for heat pump applications. Semiconductor elements, typically bismuth-telluride, can be heavily doped to create either an N-Type or P-Type and the junction therebetween will either produce heat or absorb heat, depending on the direction of, and at a rate proportional to, current passing through the junction. For practical heat pump applications, a number of semiconductor elements are combined in a module with opposite conductivity types connected electrically in series and thermally in parallel to increase heat-generating and heat-absorbing capacity. Thermoelectric modules are available commercially for refrigeration applications such as small refrigerators, water and beverage coolers, and direct temperature control in laboratory and scientific instruments. Thermoelectric modules are also commercially available for 12-volt source applications, for example, for use with an automotive battery or a 110-volt AC to 12-volt DC converter. They operate economically, are compact and lightweight yet rugged and durable, and have no moving parts. For purposes of the present invention, as will later be apparent, thermoelectric modules operated as a heat pump can selectively heat or cool one junction, depending on the direction of current flow.